This invention relates to a magnetic video signal recording and reproducing method and apparatus, especially a video signal segment recording and reproducing method and apparatus.
A high-quality television system having a far greater resolution than the NTSC and PAL television systems is being developed. Since the development aims to increase vertical and horizontal resolution to twice that of the present level, the bands of luminance or brightness signals (Y), and chrominance or chromaticity signals (C), may be as broad as 30 MHz. To transmit those signals within a band as narrow as possible, about 8 MHz, an offset sampling method has been proposed between scanning lines and frames. The offset sampling method is detailed in the Technical Report of the Television Society of Japan, TEBS 95-2, March, 1984.
A rotary head helical scanning video tape recorder (hereinafter, VTR) which records and reproduces those video signals causes problems. If the band 8 MHz is to be recorded and reproduced with small apparatus VTRs, it must, because of the limitation of the diameter of a cylinder mounting the rotary heads, adopt a "segment recording system".
There is a four-head VTR known in the broadcasting industry as the "segment recording magnetic video recording and reproducing apparatus" in which video signals are recorded on a magnetic tape by having one vertical scanning period (i.e., one field) divided into a plurality of periods. This VTR is disclosed in detail in papers such as "VTR Techniques" (edited by the Japan Broadcasting Publishing Association and the Television Institute under the supervision of Minoru Inazu and Takashi Iwasawa). Since the video signal is recorded by dividing one field into a plurality of tracks, it is essential to provide the segment recording type VTR with a time-axis correcting circuit for correcting "skew" (abrupt changes in the time axis) when the tracks are changed in a reproducing mode. "Skew" may be caused by such common problems as inaccurately mounted rotary heads or by stretching or shrinking of the magnetic tape. One skew correcting method, known in the prior art has a time-axis correcting method in which reproduced video signals are fed through variable delay lines or the like, so that their delay time varies with the extent of skew. The phases of the reproduced video signals and their horizontal synchronizing signals may be made continuous by lengthening or shortening the horizontal blanking period of the video signals or, more commonly, by changing the period of a "front porch" occurring immediately before the horizontal synchronizing signals by the extent of skew, as is detailed in the above-specified papers (especially Section 7).
In the prior segment recording art described above, the correctable skew is limited by the time width of the front porch of the video signals and is restricted to about one to two microseconds in the television systems.
In contrast, home VTRs do not use the segment recording described above; home VTRs usually employ "helical scanning", in which one field of video signals is recorded on one track. Lately the segment recording described above has been tested with home VTRs. The aim is to reduce the diameter of a rotary drum, thereby further reducing the size and weight of the VTR; to increase the rate of revolution of the rotary drum, thereby improving the picture quality; and to realize a new VTR which can record video signals having a band several times as wide as that of the prior art, for example, about 8 MHz, as in the "high-quality television" having far higher fineness and picture quality than those of the conventional television system. In the helical scanning home VTRs, however, the tolerance of a mechanical system such as a rotary head system or a tape drive system may be quite wide due to limitations imposed by production requirements; hence, the extent of skew may increase to several microseconds. If compatibility or interchangeability is taken into consideration, it becomes still more necessary to estimate the tolerance.
With the partially proposed high-quality television, on the other hand, the horizontal blanking period to be assigned to the video signals is short (e.g., one microsecond or shorter), as is disclosed in papers such as the Technical Report of the Television Society of Japan (VOL. 7, No. 44, March, 1984, "Satellite One-Channel Transmission System MUSE of High-Quality Television").
If the amount of the skew in the home VTRs is considered, therefore, in either the conventional segment recording method, or either the existing television system or the proposed high-quality television system, it has been found quite difficult to completely eliminate the skew thereby to devise a new VTR capable of achieving the aforementioned purpose in practical use.
Another example of the prior art resorting to the segment recording, is a "digital VTR", in which analog video signals are converted into digital signals and recorded as PCM signals, for example, as shown in U.S. Pat. No. 4,330,795. From the necessity for reducing the quantum error accompanying the digitalization of the video signals, the number of quantum bits is increased to dramatically raise the transmission rate of the PCM signals to be recorded in the magnetic tape so that the recording density of the tape is so reduced as to make it difficult to have sufficient recording time. Moreover, the signals occupy such a wide band as to invite technical difficulties. These and other problems have limited the use of the digital VTRs in the home.